Wow, this thread has moved fast...
I think realistically I'm not going to get the chance to go more than 3-5 times per year until my son's a bit more grown up, so I think hiring a van/car is going to be more cost effective than buying/insuring even the cheapest car. I'm not aware of any car clubs in South Manchester, but I could be wrong.
Gatley -> Peaks on the Buxton line is probably the cheapest option, but is a full day's trip (have done it recently to go to Buxton with my son).
Now, I've had a look at the sustrans site, but as far as I can see, most of the routes to Marple either require significant diversions or they are actually on road; but I might give some of the more significant diversions a try as a route to work (central stockport) once the weather improves a bit.